SHARE

## Clock

So, what is clock anyway? Clock is a signal used to sync things inside the computer. Take a look at Figure 2, where we show a typical clock signal: it is a square wave changing from “0” to “1” at a fixed rate. On this figure you can see three full clock cycles (“ticks”). The beginning of each cycle is when the clock signal goes from “0” to “1”; we marked this with an arrow. The clock signal is measured in a unit called Hertz (Hz), which is the number of clock cycles per second. A clock of 100 MHz means that in one second there is 100 million clock cycles.

Figure 2: Clock signal.

In the computer, all timings are measured in terms of clock cycles. For example, a RAM memory with a “5” latency means that it will delay five full clock cycles to start delivering data. Inside the CPU, all instructions delay a certain number of clock cycles to be performed. For example, a given instruction can delay seven clock cycles to be fully executed.

Regarding the CPU, the interesting thing is that the CPU knows how many clock cycles each instruction will take, because it has a table which lists this information. So if it has two instructions to be executed and it knows that the first will delay seven clock cycles to be executed, it will automatically start the execution of the next instruction on the 8th clock tick. Of course this is a generic explanation for a CPU with just one execution unit – modern processors have several execution units working in parallel and it could execute the second instruction at the same time as the first, in parallel. This is called superscalar architecture and we will talk more about this later.

So, what clock has to do with performance? To think that clock and performance is the same thing is the most common misconception about processors.

If you compare two completely identical CPUs, the one running at a higher clock rate will be faster. In this case, with a higher clock rate, the time between each clock cycle will be shorter, so things are going to be performed in less time and the performance will be higher. But when you do compare two different processors, this is not necessarily true.

If you get two processors with different architectures – for example, two different manufacturers, like Intel and AMD – things inside the CPU are completely different.

As we mentioned, each instruction takes a certain number of clock cycles to be executed. Let’s say that processor “A” takes seven clock cycles to perform a given instruction, and that processor “B” takes five clock cycles to perform this same instruction. If they are running at the same clock rate, processor “B” will be faster, because it can process this instruction is less time.

For modern CPUs there is much more in the performance game, as CPUs have different number of execution units, different cache sizes, different ways of transferring data inside the CPU, different ways of processing the instructions inside the execution units, different clock rates with the outside world, etc. Don’t worry; we will cover all that in this tutorial.

As the processor clock signal became very high, one problem showed up. The motherboard where the processor is installed could not work using the same clock signal. If you look at a motherboard, you will see several tracks or paths. These tracks are wires that connect the several circuits of the computer. The problem is that with higher clock rates, these wires started to work as antennas, so the signal, instead of arriving at the other end of the wire, would simply vanish, being transmitted as radio waves.

Figure 3: The wires on the motherboard can work as antennas.

## Contents

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.